The level is obviously an essential instru ment in using the leveling rod, to mark the upper limit of the vertical distances measured upon the latter. It consists of a telescope, 20 inches in length, in which the line of sight is carried by the intersection of cross-hairs, and to which is attached a °bubble tube° of glass filled with a mixture of ether and alcohol except for the long bubble of air. This tube is about eight inches long and three-fourths of an inch in diameter, and is curved to a long radius, the convexity of the curve being set uppermost. Upon this face of the tube is a centre line and graduations on either side of it. The tube is so attached to the telescope that it may be ad justed and brought accurately parallel to the line of sight through the telescope. The com bination of bubble tube and telescope is rever sible in its Y-shaped bearings, from which it is called a Y-level; and is mounted upon a frame with a long conical spindle held by a pair of plates between which are four leveling screws, the whole being attached to a tripod, as shown in the accompanying illustration. A pair of stadia wires is often added to the cross-hairs of the telescope, so that the distance of the leveling rod may be computed by talcing a sec ond reading, above or below the level line at the time when the level is fixed. An ingenious de vice renders the stadia wires invisible when the cross-hairs are being used, and vice-versa. A small pocket level known as the ((Locke Level,'" is of great usefulness in reconnaissance work. In this instrument the bubble tube is attached to • the top of the tiny telescope, the upper part of which is cut away, and a semi-circular mirror at 45 degrees placed beneath it in the telescope. The knage of the bubble in the mirror decides when the instrument is held in a true level posi tion, and at the same moment the point at which the level line strikes the leveling rod is 'in plain view.
For the measurement of angles in the field the commoner instruments are the magnetic compass and the transit. The former depends upon a continual reference of the direction of a line to the magnetic north-and-south line indi cated by a magnetic needle swinging in a com pass box, the rim of which is graduated to 15 minutes of arc; from which the angle can be very closely estimated to five minutes. By an attachment to the compass it may be made a solar compass, a necessity where the principal lines of the survey are referred to the true meridian, as in governmeat surveys of the pub lic lands. The compass may be still further improved by the addition of a telescope, which may be clamped to the rear sight, and this tele scope may have a long bubble tube, making it available for running levels, and it may also carry a vertical circle for measuring vertical angles.
The transit is the engineer's instrument of precision for angular measurements. Its essea tial factor is the plate, from five to seven inches in diameter, bearing graduations of exquisite accuracy, to 30 minutes of arc in the smaller instruments and 20 minutes in the larger (mei.
Verniers are affixed at opposite sides of the plate by which the angular measurement may be made to 30 seconds in the one case or to 20 seconds in the other. The telescope which car ries the line of sight with the intersection of cross-hairs is often fitted with stadia wires also, so that the tangents of the distances may be measured upon a leveling rod. A long bubble tube enables the engineer to use the transit for all ordinary leveling work. It is not uncom mon to have a four-indi vertical circle attached, carried on four arms, and reading to. single minutes. A solar attachment, consisting of three arcs of circles, on which the latitude of the place, the declination of the sun and the hour of the day may be set off, converts the instrument into a solar transit. Where a ver tical circle is a part of the instruznent, this serves for the latitude arc. The. transit shown in the illustration is equipped with the popular Saeginuller solar attachtnent mounted above the primary telescope. A special form of the transit for mining engineers has the axis of the telescope extended beyond the standard at one side, and a second telescope mounted upon it so that a sight may be made directly down ward. For very accurate and extended work, as in geodetical work, the telescope of the tran sit is sometimes 36 inches in length, the hod-, zontal graduated circle 36 inches in ,diameter and the vertical Circle 24 inches in diameter.
The plane table is practically a drawing board, usually 24 by 36 inches. In its simplest forrn the ruler or galidadel carries two sights at the ends by which the direction is obtained, the line of this direction being then drawn along the ruler upon a sheet of paper attached to the board. In the better class of instrument the sights are replaced by a telescope, which some times carries stadia wires for measuring dis tances, a compass box for orientation and short vertical arcs for measuring tangents which can not be covered by the stadia wires. The pipe table is of use chiefly for rapidly filling in topo graphical details upon the may made by ordi nary surveying method.s. The plane table shown in the accompanying illustration is equipped with .reels for carrying a continuous roll of drawing paper. See SURVEYING.
Bibliography.— Baker, I. O., 'Engineers' Surveying Instruments: Their Construction, Adjustment and Use' (New York 1909); Bausch and Lomb Optical Company, (Metro Manual: A Handbook for Engineers' (Roches ter 1915); Gurley, W. and L. E., (Manual of the Principal Instruments used in American Engineering and Surveying' (Troy 1914) ; Lovell, W. H., (The Plane Table, and Its Use in Surveying' (New York 1908) ; Stanley, W. F., (Surveying and Leveling Instruments' (London 1914) ; Webb, W. L., and Fish, J. C. L., 'Technic of Surveying Instruments and Meth ods' (New York 1917).